(1) The Field of the Invention
The present invention is directed to a process for the texturization of polycapronamide (nylon 6) fibres and to the texturized polycapronamide fibres thus obtained.
(2) The Prior Art
It has been theoretically known for a long time that it is possible to obtain texturized yarns by false twisting and setting synthetic yarns, either wholly preoriented by drawing or preoriented, to a more or less marked degree but not completely, by high speed spinning or partial drawing, or, further, completely unoriented, viz. as they are spun by the normal speed conventional processes.
The process involving complete drawing and subsequent texturization, by false twisting, is actually the only one, industrially applied to polyamide fibres. The process involving partial preorientation by high speed spinning and subsequent texturization by false twisting with concurrent or sequential completion of the drawing is industrially applied to polyester fibres. The texturization by false twisting starting from a completely unoriented yarn, spun at normal speed,--normal speed meaning, in the case of polycapronamide yarns to which this invention refers, speeds not greater than 1500 and preferably not greater than 1200 meters per minute--has not found application in the industry. The texturization of unoriented yarn has hitherto been believed to be in practicable for a series of reasons. It was not believed possible to carry out the operations regularly, by means of normal industrial apparatus without causing an excessive number of yarn breakages. It was also believed that the quality of the texturized yarn thus obtained would be inferior.
In this connection, the state of the art is represented, as far as the Applicant is aware, by U.S. Pat. No. 3,601,972 in the name of Rogers, wherein the texturization of polyamide yarns is described and discussed. Said patent, precisely, underlines the impracticability of a texturization starting from unoriented yarn, both with respect to the process and to the quality of the yarn, and suggests partially to predraw the yarn and then to texturize it using a ratio between the draw ratio in the texturization stage and the draw ratio in the predrawing stage which preferably varies from 0.3 to 0.8, viz. starting from a yarn having a rather marked degree of orientation. The Rogers patent describes only nylon 66, viz. polyadipate of hexamethylenediamide yarns, and the maximum draw ratio exemplified is 1:3, viz. a ratio lower than is normal for nylon 66.
An analysis of the art relative to the drawing-texturization processes, which generally declares the impracticability of the texturization by false twisting and setting of completely undrawn yarns, is contained in U.S. Pat. No. 3,771,307 as well. The patent describes a process analogous to the Rogers process applied to polyester yarns.
The fact that processes of the type described by the Rogers patent have not been practically applied to polyamides, can be explained by technical considerations. Polyester yarn is hot drawn and it is therefore logical to combine this operation with other operations which are done such as false twist texturization. Such a combination is neither logical nor desirable in the case of the polyamides which are cold drawn.
On the other hand, the drawbacks recited by Rogers with regard to the texturization of non-preoriented nylon, are connected to a substantial extent with the plasticity of nylon at the false twist setting temperatures. If it is taken into account that Rogers has found said drawbacks when using nylon 66, the persons skilled in the art could not but believe that the results would be even worse with nylon 6, the melting point of which is considerably lower than that of nylon 66.
Summing up, the state of the art indicated that the texturization by false twisting and setting of unoriented synthetic fibre was not applicable industrially to polyamide fibres because of the serious operational difficulties and of the low quality of the product that would theoretically be obtained. The prior art suggested that said drawbacks, which mitigated against use of the process for nylon 66, should be even more serious for nylon 6 (polycapronamide, normally obtained by polymerization of caprolactam).